1. Field of the Invention
The present invention relates generally to the field of radar systems and more particularly, but not by way of limitation, to an improved continuous wave (CW) Doppler radar system with leakage cancellation.
2. Prior Art
CW Doppler radar systems offer a number advantages over pulse radar systems. Even so, CW Doppler radar systems have always suffered from a disadvantage of feed-through or leakage signals in the receiver. The leakage signal is usually several orders of magnitude larger than the received signal and tends to hide the received signal. The leakage signal is created because of imperfect isolation between the transmit and receive paths of a CW Doppler radar system and as well as scattering from nearby objects. In The Electrical Engineering Handbook published by the CRC PRESS in 1993, Section 39.2 relating to Continuous Wave Radar noted that the largest disadvantage for CW radars is the need to provide antenna insolation (reduce spillover) so that the transmitted signal does not interfere with the receiver. A second disadvantage noted was the existence of noise sidebands on the transmitter signal which reduce sensitivity because the Doppler frequencies are relatively close to the carrier signal.
One solution has been to separate the receive antenna from the transmit antenna. This provides better isolation between the transmit and receive paths. However, even relatively small side lobes in the transmit and receive antennas will result in coupling that allows leakage signals to occur.
An example of this solution is U.S. Pat. No. 3,978,483 issued on Aug. 31, 1976 to Bernard L. Lewis et al and which is discloses adaptive canceler loops that receive inputs from an auxiliary channel signal and operate on a main channel signal to reduce interference in the main channel signal. The auxiliary channel signal is supplied through a quadrature hybrid circuit to provide I and Q components to the adaptive loops, which loops are connected in series and cascaded such as to form a plurality of serially connected I and Q loops.
Another solution has been to use an auxiliary antenna to also receive the return signal. By subtracting the signal received by the primary receive antenna from the signal received by the auxiliary receive antenna, the leakage signal is left. This leakage signal is subtracted from the return signal in the primary antenna, thus reducing the leakage signal. However, this system requires at least two antennas and makes the system bulky and expensive. Examples of prior art patents that disclose the use of a plurality of auxiliary receiving antennas are U.S. Pat. Nos. 4,439,770 and 4,689,628. U. S. Pat. No. 4,439,770 issued on Mar. 27, 1984 to Bernard L. Lewis et al for CASCADED ADAPTIVE LOOPS and discloses a sidelobe-canceler system for canceling jamming interference signals from a radar signal that includes serially cascaded cancellation channels utilizing preprocessing cancelers and main-channel cancelers. U.S. Pat. No. 4,689,628 issued on Aug. 25, 1987 to Bernard L. Lewis for ADAPTIVE SIDELOBE CANCELER SYSTEM discloses the use of a plurality of auxiliary antennas that receive interference from a plurality of sources and provide independent inputs to a configuration of adaptive canceler loops. In a canceler system each of the independent loop outputs are then used as an input to a canceler loop in the main channel to cancel interference in the receiving system.
A single antenna system having provision for leakage cancellation is discussed in "Some Advances in CW Radar Techniques" by J. D. Harmer and W. S. O'Hare, IRE 5th MIL-E-CON Record, 1961, pages 311-323. This paper discusses an intermediate frequency (IF) feedback circuit that cancels the leakage signal. However, this circuit only works for a single frequency CW radar system. In addition, the circuit requires several high precision oscillators and these oscillators must constantly be calibrated to each other. Failure to continually calibrate the circuit can result in accidentally canceling the desired signal.
Thus, a need exists for a poly-frequency CW Doppler radar system having provision for leakage cancellation that does not require high precision, high cost oscillators and that operates with a single antenna for transmitting and receiving radar signals and with multiple carrier frequency signals.